Inorg. Chem. 1988, 27, 3698-3700
3698
Contribution from the Department of Chemistry and Quantum Institute, University of California, Santa Barbara, California 93 106, and Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Pressure Effects on Copper(I) Complex Excited-State Dynamics. Evidence Supporting an Associative Nonradiative Deactivation Mechanism Daniel R. Crane,' John DiBenedetto,*,' Cynthia E. A. Palmer,* David R. McMillin,*,* and Peter C. Ford*,' Received April 14, 1988 The effects of hydrostatic pressure on the metal to ligand charge-transfer (MLCT) excited-state emission lifetimes for the copper(1) dpp = 2,9-diphenyl-l,lO-phenanthroIine)have complexes Cu(dmp),+ and Cu(dpp),+ (dmp = 2,9-dimethyl-l,lO-phenanthroline, been determined in CH2C12solution. In the absence of added quenchers, volumes of activation AV,' for the nonradiative decay pathway have been determined to be -3.4 and -1.6 cm3/mol, respectively. For the previously reported quenching of [Cu(dmp),f]* by the cosolvents CH$N and CH30H, AV,* values of -6.2 and -5.4 cm3/mol were measured. These data are consistent with the proposal that an associative mechanism plays a role in the nonradiative deactivation of the MLCT excited state of the less sterically crowded (dmp), complex. Introduction Investigations of the effects of hydrostatic pressure (P)on metal complex excited-state (ES) dynamics have provided valuable insight into the mechanisms of relevant ES mechanism^.'^ O n e major advantage is that P can be varied continuously over large ranges, thus providing a systematic perturbation of the medium. The present study focuses on the ES dynamics of the copper(1) complexes Cu(dmp),+ ( A ) and Cu(dpp),+ (B) ( d m p = 2,9-dim e t h y l - 1, l o - p h e n a n t h r o l i n e , dpp = 2,9-diphenyl- 1,lOphenanthroline), each of which displays ambient-temperature luminescence from lowest energy, triplet metal t o ligand charge-transfer ( M L C T ) states in fluid CH2C12 ~ o I u t i o n s . ~ - ~ Subsequent studies have revealed that the MLCT emission from Cu(dmp),+ solution but not t h a t from Cu(dpp),+ solution is quenched by the presence of various Lewis bases including the donor solvents CH$N and CH30H.89 The mechanism proposed for this process involves an association between the MLCT ES of the former ion and one of these donors to form a nonemissive five-coordinate ES intermediate that decays more rapidly than the original MLCT state. The present investigation was initiated on the anticipation that such a pathway should display a significant dependence on the hydrostatic pressure. Experimental Section Materials. The complex salts [Cu(dmp),]BPh4 and [Cu(dpp),]BPh4 were prepared according to published The solvent CH2C12 (Fisher ACS grade) was washed with concentrated H2S04,dried over CaCI,, and then distilled from LiA1H4 or passed through a column of activated alumina. Acetonitrile (Burdick and Jackson High Purity grade) and methanol (Fisher ACS Spectroanalyzed grade) were used without further purification. Sample Preparation for High-pressure Luminescence Experiments. Methylene chloride solutions of the copper(1) salts were prepared at University of California. Purdue University. (a) Weber, W.; van Eldik, R.; Kelm, H.; DiBenedetto, J.; Ducommun, Y.;Offen H.; Ford, P. C. Inorg. Chem. 1983,22,623-628.(b) Weber, W.; DiBenedetto, J.; Offen, H.; van Eldik, R.; Ford, P. C. Inorg. Chem. 1984,23, 2033-2038. (c) DiBenedetto, J.; Watts, R. J.; Ford, P. C. Inorg. Chem. 1984, 23, 3039-3040. (d) DiBenedetto, J.; Arkle, V.; Goodwin, H. A.; Ford, P. C. Inorg. Chem. 1985,24,455-456. DiBenedetto, J.; Ford, P. C. Coord. Chem. Reu. 1985, 64, 361-382. Ford, P. C. In Inorganic High Pressure Chemistry, Kinetics and Mechanisms; van Eldik, R., Ed.; Elsevier: Amsterdam, 1986;Chapter 6,pp 295-338. (a) Blaskie, M. W.; McMillin, D. R. Inorg. Chem. 1980,19, 3519-3522. (b) Kirchhoff, J. R.; Gamache, R. E.; Blaskie, M. W.; Del Paggio, A. A.; Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983,22, 2380-2384. (a) Dietrich-Buchecker, C. 0.;Marnot, P. A.; Sauvage, J. P.; Kirchhoff, J. R.;McMillin, D. R. J . Chem. Soc., Chem. Commun. 1983,s13-5 15. (b) Ahn, B. T.; McMillin, D. R. Inorg. Chem. 1981,20, 1427. (a) McMillin, D.; Kirchhoff; J. R.; Goodwin, K. V. Cmrd. Chem. Reu. 1985,64 83-92. (b) Goodwin, K. V.; McMillin, D. R. Inorg. Chem. 1987,26, 875-877. Palmer, C. E. A.; McMillin, D. R.; Kirmaier, C.; Holton, D. Inorg. Chem. 1987,26,3167-3170.
0020-1669/88/1327-3698$01.50/0
20-50 NM concentrations for experiments in the absence of quenchers and at 40-90 WMfor experiments in the presence of the added quencher acetonitrile or methanol. The solutions were degassed by bubbling with dinitrogen in a glovebox filled with N2.Exact solution concentrations were determined after preparation and degassing by measuring the absorption spectrum on a Cary 118 spectrophotometer (A, e454nm= 7950 M-l cm-1; B, = 3230 M-l cm-'). Degassed acetonitrile or methanol was then added quantitatively via syringe in the glovebox. The resulting solution (about 1 mL) was loaded into a small glass capsule (25 mm in length, 8 mm in diameter) that was subsequently capped with a Teflon piston with two Viton O-rings. (The capsule/piston combination was designed to transmit the applied pressure without exchanging solution with the surrounding medium.) The capsule was placed into a modified Nova-Swiss four-window, 400-MPa (4-kbar) high-pressure spectroscopic cell. The high-pressure cell was then filled with degassed pressuretransmitting fluid (pure CH2C1, or CH,Cl2 with the appropriate concentration of quencher) and sealed. Apparatus for Lifetime Measurements under Pressure. The highpressure cell loaded as described was attached to an Enerpac hand pump and gauge, which were used to generate and measure the applied press u r e ~ .The ~ apparatus used for lifetime measurements was based on a Quanta Ray DCR-1A Nd/YAG pulse laser with harmonic generator operating at 532 nm as the excitation source. The emission was monitored at right angles at 670 nm with an RCA 8892 or EM1 9816A PMT through a Spex single or double monochromator. The signal from the PMT was processed by a Tektronix 7912 AD transient digitizer and a Tektronix 4052 mi~rocomputer.~ Experimental procedures for Lifetime Measurements. All experiments were carried out at room temperature (23 f 1 "C). For a particular pressure run, three to six luminescence lifetime measurements were performed at each pressure. A period of approximately 10 min was allowed for equilibration subsequent to changing pressure in the cell. Lifetime measurements were generally made over the following approximate pressure sequence in order to check for any hysteresis (there was none in the reported data): (1) ambient pressure, (2) 100 MPa, (3) 200 MPa, (4)300 MPa, (5) 200 MPa, (6)250 MPa, (7) 100 MPa, (8) 150 MPa, (9)100 MPa, (10)50 MPa, (11) ambient pressure.
Results and Discussion The copper(1) complexes Cu(dmp)2+ ( A ) and Cu(dpp),+ (B) in ambient-temperature dichloromethane solutions display emission spectra that have been assigned as luminescence from d-r* MLCT states, the lowest ES being largely triplet in c h a r a ~ t e r . 6 , ~ *While ~,'~ there is some evidence, based on temperature-dependent luminescence supporting contribution to the emission from a higher energy singlet MLCT state, t h e independence of t h e measured emission lifetime (T) over the same temperature regime indicates that T is determined by decay processes from the triplet. Steric effects on ES lifetimes are significant, with B having a substantially longer emission lifetime than A. Furthermore, the 2,9-dimethylphenanthrolinecomplex A has been shown to be subject to a second-order quenching by Lewis bases, while B, with the more sterically demanding 2,9-diphenylphenanthrolineligand, (10) Ichinaga, A. L.; Kirchhoff, J. R.; McMillin, D. R.; Dietrich-Buchecker, C. 0.;Marnot, P. A.; Sauvage, J.-P.Inorg. Chem. 1987,26,4290-4292.
0 1988 American Chemical Society
Inorganic Chemistry, Vol. 27, No. 21, 1988 3699
Cu(1) Complex Excited-State Dynamics Table I. Photophysical Data for the Complexes Cu(dmp),+ and Cu(dpp),' in CH2Cll Solution" Cu(dmD),+ No Added Quencher ~ ( 0 . 1MPa)b 90 T ( 150 MPa) 70 ~ ( 3 0 0MPa) 58 AV,' -3.4 & 0.2 (4)
I
1
Cu(dpP),+
1
~~
0.30 M CHjCN 63 50 39 -4.3 f 0.2 (2)
~ ( 0 . 1MPa) ~ ( 9 1MPa) ~ ( 2 6 4MPa)
AVn*(aPP)
260 246 227 -1.2 f 0.2 (2)
0.30 M CH,OH 66 52 42 -4.0 f 0.2 (2)
~ ( 0 . 1MPa) T ( 105 MPa) r(264 MPa)
AV,*(app)
258 23 1 21 1 -1.6 & 0.2 (5)
I
NA
T
"BPh,-; = 23 & 1 OC. b~ values in ns. ,A,,* values in cm'/mol; number of independent pressures is in parentheses.
is not similarly quenched.*q9 It has been proposed that the quenching mechanism involves association of the Lewis base Q with the formally d9 "Cu(I1)" metal center of the MLCT state to form a short-lived, pentacoordinate excited-state complex that decays rapidly by an independent nonradiative pathway (Scheme I).8 Notably, any MLCT transition should enhance a metal center's reactivity toward nucleophiles;" however, this tendency should be accentuated for Cu(I), given that Cu(I1) generally has a higher coordination number. An associative character of a contributing pathway to the nonradiative decay of Cu(1) MLCT states would also be consistent with 3A* having a shorter lifetime (90 ns) than 3B* (260 ns) in CH2CI2,the nonemissive character of the parent complex C ~ ( p h e n ) ~ +and , l ~the observation of MLCT emission from several other Cu(1) complexes constructed with ligands designed to minimize interactions of the metal center with the solvent.12 Scheme I [Cu(dmp),+]*
+Q
[Cu(dmp),+]*
k,
[Cu(dmp),+]* [Cu(dmp),Q']*
k,'
2[ C ~ ( d m p ) ~ Q + l *
-
C ~ ( d m p ) ~++hv
kn
Cu(dmp),+
Cu(dmp),Q+
fast
Cu(dmp),+
Pressure
1
IMPaI
Figure 1. Pressure dependences of the MLCT lifetimes of Cu(1) complexes in CH2CI2solution (23 f 1 "C). Pluses (+) represent Cu(dmp),'; stars (*) represent Cu(dpp),+.
+Q
The application of pressure to CHzC12solutions of either A or B led to systematic decreases in the emission lifetimes. Table I summarizes some lifetime data at ambient pressure and at 300 MPa, while Figure 1 displays graphically this effect for each as plots of In ( T " / T ) vs P (where T" is the lifetime for P = 0.1 MPa). From these data, it is clear that the emission lifetimes for 3A* are significantly the more pressure sensitive. For emission from a single ES or collection of thermally equilibrated ES's, the measured luminescence lifetime T is the inverse of the total decay rate constant kd: T = kd-' (1) where kd is the sum of the radiative ( k r ) , reaction ( k J , and nonradiative (k,) rate constants for ES deactivation. For the present case, the emission quantum yields at ambient temperature are small (510-3),'0and unimolecular photoreactions are not observed; hence, the dominant deactivation pathway is nonradi-
(1 1) Ford, P. C.; Wink, D. A,; DiBenedetto, J. Prog. Inorg. Chem. 1983,30, 213-272. (12) (a) Sakaki, S.; Koga, G.; Ohkubo, K. Inorg. Chem. 1986, 25, 2330-2333. (b) Kern, J.-K.; Sauvage, J.-P. J . Chem. SOC.,Chem. Commun. 1987, 546. (c) Sakaki, S.; Koga, G.; Hinokuma, S.; Hashimoto, S.;Ohkubo, K. Inorg. Chem. 1987,26, 1817-1819. (d) Sorrell, T. N.; Borovik, A. S. Inorg. Chem. 1987, 26, 1957-1964.
ative, and the pressure sensitivity of T reflects that of the various nonradiative deactivation pathways. The volume of activation AK* for a dynamic process characterized by the rate constant ki is defined by the relationship5 d(ln k i ) AK' = - R 7 ( d P )
T
Thus the AVn% for the nonradiative decay of 'A* and 3B* in CH2C12in the absence of added quenchers are proportional to the slopes of the plots in Figure 1 (Le., AVn* = -RT X slope). From such slopes were calculated the average AV,* values -3.4 f 0.2 and -1.6 f 0.2 cm3/mol for A and B, respectively (Table 1). Consistent with earlier report^,^^^ the addition of 0.3 M acetonitrile or methanol shortened the observed emission lifetime of 'A* by 25-30%, while the addition of 0.3 N CH3CN had no effect on that of 'B* (Table I). In pure acetonitrile, the luminescence lifetime for 'B* was considerably shorter (90 ns) than in pure dichloromethane (260 ns), while that for 'A* has been shown to be too short to measure with the present apparatus (2 n ~ ) . ~ The "quenching constant" k, (see below) for the effect of the added cosolvents in CH2C12can be calculated from the equation T - l - (p)-1 k, = (3) [Ql where T is the lifetime at a certain pressure P,T" is the lifetime in the absence of quencher at the same pressure, and [Q] is the concentration of the quencher. From these limited data, the calculated k;s are 1.6 X lo7 and 1.4 X lo7 M-' S-I, for quenching of 3A*by CH3CN and CH30H, respectively, at ambient pressure. These values are within experimental uncertainties of those reported b e f ~ r e .No ~ value for k, can be calculated for 3B*, since quenching was not observed for 0.3 M CH3CN in CH2C12solution and since the effect of using CH3CN as the pure solvent includes global effects separate from any bimolecular quenching mechanism. Simple plots of In ( T " / T ) vs P for the above CH2Clzsolutions of A and of B give Ap(apparent) values. These are respectively -4.3 f 0.2 and -4.0 f 0.2 cm3/mol for the luminescence decay of 'A* in 0.3 M CH3CN and in 0.3 M CH'OH, both more negative than in neat CH2CI2. In contrast, for 'B* the AV(apparent)'s are -1.2 & 0.2 cm3/mol in 0.3 M CH3CN in CH2C12 solution and -1.4 0.2 cm3/mol in neat CH3CN, both values marginally more positive than in neat CH2C1,. The Ap(apparent) values determined above include contributions from several terms (Le., kd = k, + k,[Q]). The quenching constants k, can be separately calculated at each pressure at which T was determined according to eq 3, and the AV,* values can be determined from plots of In (k,"/k,) vs P,e.g., Figure 2. There is much greater scatter in these plots than in Figure 1 as well as the suggestion of some curvature; however, it is clear that the k:s
*
3700 Inorganic Chemistry, Vol. 27, No. 21, 1988
Crane et al.
Table 11. AVi* Values for Cu(dmp),+ and Cu(dpp),+"
soh Cu(dmp),+ in CH2Clz Cu(dmp),+ in 0.3 M CH3CN/CH2C12 Cu(dmp),+ in 0.3 M CH30H/CH2CI, Cu(dpp),+ in CH2C12 Cu(dpp),+ in 0.3 M CH3CN/CH2C12 Cu(dpp),+ in CH3CN
AVd*b -3.4 f 0.2 -4.0 f 0.2 -4.0 & 0.2 -1.6 f 0.2 -1.4 f 0.2 -1.2 f 0.2
AVq*
AVn*
-3.4 -3.4 -3.4 -1.6 -1.6 -1.2
-6.2 f 0.5 -5.4 f 0.5
"Determined at 23 f 1 "C; BPhc salts in each case; AV values in cm3/mol. 'Determined from the slopes of In ( T " / T ) vs P plots. CTheA v l value in neat solvent. dCalculated from slopes of In (k,"/k,) vs P plots; k , calculated according to eq 3 .
.50
.t1
.LO
50 0
150.
Pressure
250
350
'
450
'
~
[MPal
Figure 2. Pressure effects on the rate constant k, for the quenching of the MLCT state of C~(drnp)~' by methanol (0.30 M) in dichloromethane solution (23 f 1 "C).
for 3A*are considerably more pressure sensitive than are the k i s . The Ai',* values determined from the slopes were -6.2 f 0.5 cm3/mol for 0.3 M CH3CN and -5.4 f 0.4 cm3/mol for 0.3 M CH30H. The A P values summarized in Table I1 illustrate the contrasting responses to pressure of the excited-state decay rates for Cu(dmp),+ and C ~ ( d p p ) ~ + Even . in neat CH2C12,the AV,*'s are substantially more negative for 3A* than for 3B*. The small negative value of AV,* for 3B* is within the range of those seen in this laboratory for other metal complex excited states decaying nonradiatively via a unimolecular weak coupling m e ~ h a n i s m . ~ J ~ The more negative AV,* for 3A* is outside this range and suggests participation of an associative pathway as a component of the nonradiative deactivation for the less sterically demanding Cu(dmp),+ ion, Le., that the k, pathway in this case may involve (13) Fetterolf, M.; Friedman, A. E.; Yang, Y.-Y.; Offen, H.; Ford, P. C. J. Phys. Chem., in press.
formation of a solvent complex with the ES prior to nonradiative decay. However, since there is no indication that 3B* undergoes a similar mechanism for deactivation, it is unlikely that 3A* decays solely via an associative pathway. The differences between the ES properties of A and B are further accentuated when the activation volumes for the k,'s are considered. For 3A*, the AV*'s for k, are substantially more negative than those for kd in neat dichloromethane. In contrast, AVd* values for 3B* in neat CH2C12,in 0.3 M CH3CN in CH2C12, and in neat CH3CN are identical within experimental uncertainty. Although the partial molar volumes of neither the excited state 3A* nor the putative excited-state complex [Cu(dmp),Q] * are known, one may safely assume that the latter will be smaller than the sum of the Ps for 3A* plus Q. Thus an associative pathway as illustrated in Scheme I, where k, = K,k,', should display a negative A P . The magnitude of the negative Ai',* for 3A* in CH2C12plus the even more negative values of AV,* confirm the associative nature of major contributions to the nonradiative deactivation mechanism from the MLCT excited state of Cu( d r n ~ ) ~as+suggested in Scheme I. Correspondingly, the medium-insensitive, small negative values of AV,* for 3B* confirm the role of the sterically bulky 2,9-diphenylphenanthrolineligand in blocking access of various bases to metal coordination sites of the MLCT excited state.I4 Acknowledgment. This work was supported by National Science Foundation grants to D.R.M. (CHE87-19538) and to P.C.F. (CHE87-2256 1). ~
~~~
~~~
(14) A reviewer has suggested that the dynamic quenching mechanism in-
volves a selective outer-sphere solvation of the more polar quencher molecules with the MLCT excited state of Cu(dmp),+, which would also be enhanced by higher pressures. While this possibility is difficult to exclude, it seems unlikely that such interactions would be specific to the dmp complex and that the phenyls of dpp would prevent such a mechanism, unless the interaction involved the metal center. Furthermore, it should be noted that neither the ground-state absorption spectra nor the s h a p and positions of the MLCT emission bands of Cu(dpp),+ and C ~ ( d m p ) ~in+ CH,CI, solution are affected by addition of 0.3 M CH9CN.
